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The methods currently employed for seagrass production estimates include biomass, marking and metabolic techniques, the latter including the O-2 and C-14 methods. Both are currently in general use and have generated a great deal of seagrass-production data. In this work, we measured carbon incorporation rates in Zostera marina L. using the stable carbon isotope C-13 as a metabolic tracer. We tried this method in an attempt to overcome the 2 major methodological problems of the O-2 and C-14 methods, i.e. the limitations when measuring low production rates and the hazardous and laborious handling of radioactive isotopes, respectively. To validate the C-13 method we compared it with the classical O-2 and C-14 methods in microcosm experiments. The 2 carbon-tracer techniques were in overall good agreement (C-13 = 0.12 + 1.03 X C-14; R-2 = 0.964, p <0.001, n = 10). Production rates derived from the C-13 method were on average 1.34 +/- 0.03 (n = 42) times higher than those obtained with the O-2 method (C-13 = 0.99 + 1.15 X O-2; R-2 = 0,788, p <0.001, n = 42), suggesting that for short incubation lengths (in this study between 0.5 and 4.0 h) the C-13 method provides estimates very close to gross production. The pros and cons of all 3 methods are critically discussed. [KEYWORDS: seagrass production; Zostera marina; stable carbon isotopes; O-2 method; C-14 method; C-13 method Zostera-marina l; saline lake grevelingen; photosynthetic utilization; carbon uptake; phytoplankton; eelgrass; community; growth; light; macrophytes]